1. Field of the Invention
The present invention is directed to a self-sustained plasma system and method and, in particular to a non-thermal plasma apparatus using a capillary electrode discharge configuration for the scattering, absorption, and/or reflection of electromagnetic radiation, and a process for using the same.
2. Description of Related Art
Plasma is a term used to denote a region of ionized gas. Plasma can be created through bulk heating of the ambient gas (as in a flame) or by the use of electrical energy to selectively energize electrons (as in electrical discharges). Non-Thermal Plasma (NTP) is ionized gas that is far from local thermodynamic equilibrium (LTE) and characterized by having electron mean energies significantly higher than those of ambient gas molecules. In NTP, it is possible to preferentially direct the electrical energy in order to produce highly energetic electrons with minimal, if any, heating of the ambient gas. Instead, the energy is almost entirely utilized to directly excite, dissociate and ionize the gas via electron impact.
There are many different classifications or types of plasma. The present invention is directed to a particular type of plasma referred to as the cold collisional plasma regime. In this regime the temperature of the free electrons in the plasma is about the same as the temperature of the host, background gas. These free electrons interact with the electromagnetic field of the electromagnetic waves. Energy from the electromagnetic field is absorbed by the free electrons and converted into kinetic energy. When the energetic electron collides with a molecule or atom in the background gas, the energy is transferred as heat. The heat capacity of the background gas is sufficient to absorb this heat without an appreciable rise in temperature.
A cold collisional plasma model is used to describe the interaction between the free electrons and the electromagnetic waves. The dispersion relation governing the propagation of electromagnetic waves through the plasma is represented by equation (1) as
                    k        =                              ω            ⁢                          ɛ                                c                                    (        1        )            where k is the complex wave number, ω is the angular frequency, c is the speed of light in vacuum, and ∈ is the complex dielectric constant. The equation that governs the dielectric constant is
                    ɛ        =                  1          -                                                    n                c                            ⁢                                                ⅇ                  2                                /                                  m                  e                                            ⁢                              ɛ                0                                                    ω              ⁡                              (                                  ω                  -                                      i                    ⁢                                                                                  ⁢                    υ                                                  )                                                                        (        2        )            where ne is the electron density, e is the electronic charge, me is the mass of the electron, ν is the collision frequency of the electrons with the host gas, ω is the angular frequency, and ∈0 is the complex dielectric constant. Assuming that the electromagnetic field is proportional to exp[−i(ωt−kz)], the plasma will have an absorption constant α ofα=2Im(k)  (3)where k is the complex wave number and Im(k) is the imaginary component of the wave number.
Thus, the intensity of the electromagnetic waves incident on a plasma decreases by a factor of
  1  eafter traveling a distance L through the plasma. Electromagnetic waves traveling through a plasma region over a distance L will be attenuated by the amount given in equation (4) asA(L,α)=4.34αL dB  (4)
When the frequency of the electromagnetic waves lies in the region where ω<υ and ων<nee2/meεo, the absorption coefficient α can be approximated by the equation
                    α        ≈                                            n              e                        ⁢                          ⅇ              2                                                          cvm              e                        ⁢                          ɛ              o                                                          (        5        )            The absorption coefficient α does not depend on the frequency of the electromagnetic waves over the specified range of validity of equation (5). Instead, the absorption coefficient α is broadband and depends on the charge density ne and the collision frequency ν.
If the collision frequency is relatively small and the electron density is not too large then the plasma acts as a mirror and reflects incident electromagnetic waves. More precisely under the conditions where ω>>υ and ω<√{square root over (nee2/meεo)} the reflectivity of the plasma region approaches unity. It is under these conditions that the plasma blocks or reflects substantially all incident electromagnetic waves. Under all other conditions the amount or level of reflection is less than 100% so some or all incident electromagnetic waves are absorbed.
Other work in this area includes U.S. Pat. No. 5,594,446 to Vidmar, et al., entitled, “Broadband Electromagnetic Absorption via a Collisional Helium Plasma,” which discloses a sealed container filled with Helium in which a non-self-sustained plasma is generated using a plurality of ionization sources, for example, electron-beam guns, as an electromagnetic anechoic chamber. This apparatus is limited in that it requires the use of a sealed container and is limited to use with Helium.
It is therefore desirable to develop a system and method for absorbing or scattering of electromagnetic waves that solves the shortcomings of conventional prior art systems and methods, such as being self-sustaining, that is, not requiring an external means of generating electrons lost through recombination processes, negative ion formation, etc., other than the electric field applied to maintain its equilibrium state. Such external means may include but are not limited to an electron gun, a photo-ionizing source, etc. Furthermore, it is also desirable for the improved system to be more energy efficient, operable under ambient pressure and temperature, and operable with a variety of gasses without requiring a sealed vacuum environment.